BRUSSELS — Eight armed and masked men made a hole in a security fence at Brussels’ international airport, drove onto the tarmac and snatched millions of dollars’ worth of diamonds from the hold of a Swiss-bound plane without firing a shot, authorities said Tuesday.

The gang used two vehicles in their daring raid Monday, grabbed the cache of stones and sped off into the darkness, said Anja Bijnens, spokeswoman for the Brussels prosecutor’s office.

Police found a burnt-out vehicle close to the airport later Monday night and said they were still looking for clues.

The heist was estimated at some $50 million in diamonds, said Caroline De Wolf of the Antwerp World Diamond Centre.

“What we are talking about is obviously a gigantic sum,” De Wolf told VRT network.

An airport spokesman said the robbers made a hole in the perimeter fence and drove up to the Swiss passenger plane, which was ready to leave. The robbers got out of the car, flashed their weapons and took the loot from the hold, said airport spokesman Jan Van Der Crujsse. Without firing a shot they drove off through the same hole in the fence, completing the spectacular theft within minutes, he said.

Van Der Crujsse could not explain how the area could be so vulnerable to theft. “We abide by the most stringent rules,” he said.

The Swiss flight, bound for Zurich and operated by Helvetic Airways, was canceled. Swiss, an affiliate of Germany’s Deutsche Lufthansa AG, declined to comment on the heist, citing the ongoing judicial investigation.

The insurance for air transport — handled sometimes by airlines themselves or external insurance companies — is usually relatively cheap because it’s considered to be the safest way of transporting small high value items, logistics experts say.

Unlike a car or a truck, an airplane cannot be attacked by robbers once it’s on its way, and it is considered to be very safe before the departure and after the plane’s arrival because the aircraft is always within the confines of an airport — which are normally highly secured.

Philip Baum, an aviation security consultant in Britain, said the robbery was worrying — not because the fence was breached, but because the response did not appear to have been immediate. That, he said, raised questions as to whether alarms were ringing in the right places.

“It does seem very worrying that someone can actually have the time to drive two vehicles onto the airport, effect the robbery, and drive out without being intercepted,” Baum said.

That amount of time would also allow someone to board the plane, he said.

A decade ago the Belgian city of Antwerp, the world capital of diamond-cutting, was the scene of what was probably one of the biggest diamond heists in history, when robbers took precious stones, jewels, gold and securities from the high-security vaults at Antwerp’s Diamond Center, yielding loot that police in 2003 estimated to be worth about $100 million.

Antwerp’s Diamond Center stands in the heart of the high-surveillance diamond district where police and dozens of cameras work around the clock, and security has been beefed up further since the spectacular 2003 robbery.